Satellite digital audio radio service (SDARS) is a satellite broadcast service recently approved by the U.S. Federal Communications Commission (FCC) which provides satellite transmission of digital audio programs to compatible radio receivers. The radio receivers can be stationary or mobile and are generally configured to receive signals from satellites as well as terrestrial repeaters.
Currently, existing SDARS automotive antenna modules are dual-arm modules: one designed to receive terrestrial (TER) signals and the other designed to receive satellite (SAT) signals. These dual-arm modules comprise two passive antenna elements, two low noise amplifiers (LNAs), and two radio frequency (RF) cables.
Recently, single-arm automotive roof-mount antennas have been developed. These are patch antennas which are ground dependent, i.e., they must be placed on a metallic surface of dimensions of at least ten times the size of the antenna footprint area for acceptable performance in SDARS applications. These patch antennas, when placed at a proper location on a vehicle roof, have acceptable gain at the horizon (for receiving TER signals) and acceptable gain between 20 and 90 degree elevation angles (for receiving SAT signals). As a result, new single-branch receivers are now being designed resulting in a lower receiver/antenna cost.
There is a need then, for single-arm mast-type (ground-independent) antennas. These types of antennas can be used in the place of dual-arm glass-mount and mast SDARS antennas.
A typical mast-type ground-independent antenna used in SDARS applications, is a printed quadrifilar antenna which consists of four helices spaced equally and circumferentially on a cylinder. FIG. 1 [from reference: “Combination linearly polarized and quadrifilar antenna,” A. Petros, U.S. Pat. No. 6,483,471] shows such a quadrifilar antenna consisting of four helical elements and feed network printed on a flexible substrate. As discussed in Antenna Engineering Handbook by Richard C. Johnson and Henry Jasik, pp. 13-19 through 13-21 (1984), a quadrifilar helix (or volute) antenna is a circularly polarized antenna having four orthogonal fractional turn helixes excited in phase quadrature. Each helix is balun-fed at the top or bottom with four helical arms of wires or metallic strips of resonant lengths (l=.lambda./4, m=1, 2, 3, . . . ) wound on a small diameter with a large pitch angle.
One embodiment of the novel antenna structure is shown in FIG. 2. It is a combination of quadrifilar antenna and substantially parallel and substantially concentric metallic rings positioned along the longitudinal axis of the quadrifilar antenna. This antenna is capable of efficiently receiving both satellite and terrestrial signals. FIGS. 3 and 4 show additional embodiments of the present invention according to FIG. 2. FIGS. 5 and 6 show alternative embodiments of the novel antenna in accordance with the teachings of the present invention. The quadrifilar antenna elements and rings are arranged on cylindrical structures. These structures are in turn arranged to provide a novel antenna structure of the same radiation properties as the novel antenna structure of FIG. 2. As shown in FIGS. 7 and 8, the radiation pattern of the novel antenna shows improved performance on both SAT and TER cases over the standard quadrifilar antenna. This novel antenna then is an ideal structure for use in SDARS applications.
An additional benefit of the technique presented here is that it yields lower profile antennas. The height of antennas produced using this technique, is reduced by approximately 15%.